We have isolated mutants in E. coli that affect precise excision of the transposons Tn5 and Tn10. We are characterizing three such classes at present. (1) Uex: This is a previously underscribed gene in E. coli that is 85% linked to pyrD. (2) F Factor genes: These are part of the tra operon. (3) A dna mutant, that maps at 8.5 min. A second project involves isolation and characterization of E. coli mutants in which the frequency of transposition of Tn5 and Tn10 is reduced. These fall into two classes 1.) Mutants in the po1A gene. A new type. 2.) A mutant closely linked to ribose. This one is also defective in recA dependent recombination when both parentals are double stranded DNA. Our work is to identify the proteins and enzymic activities affected by these mutants.